The present invention relates to the field of timing in a packet switched network, in particular an ATM network.
The timing of a digital transmission network is in most applications highly important. It is essential that the transmitting part and the receiving part of the transmission system use the same clock frequency and are in phase with each other. In order to obtain this, some form of synchronization is needed for most applications.
One application which does not need any synchronization is the ATM (Asynchronous Transfer Mode) transmission network. However, many devices or apparatuses connected to an ATM network require some form of synchronization, such as telephone conversation or a video conference.
Furthermore the transmission network used can also involve other transmission systems than the ATM network, such as the Public Switched Telephone Network (PSTN), which is often a synchronized network consisting of e.g. the Plesiochronous Digital Hierarchy (PDH) and the Synchronous Digital Hierarchy (SDH). Thus, the interface between the ATM network and the PSTN must provide a synchronization of the regenerated user data used by the ATM network.
The ATM standard states that the synchronization of the regenerated user data, which are transmitted from a synchronized source or transmitter, is obtained either by means of the Adaptive mechanism or the time stamping mechanism, both described in the existing standard for ATM Adaption layer used for circuit emulation (AAL1 in I363.1).
It is an object of the present invention to provide a device and a method, which accomplish a less complicated and less costly timing regeneration at the interface between a switched packed network, in particular an ATM network, and a synchronized network.
This object is obtained by means of providing a clock signal at said interface directly from the synchronized network. Thus, the synchronization is maintained. This is obtained in the following manner.
The clock of a synchronized master unit is recovered by a slave unit, e.g. by means of the adaptive mechanism. The slave unit uses the recovered clock for transmission over the packet switched network. The depacketizing at the master side of the system is then performed using a clock signal provided directly from the synchronized network, i.e. without recovering the clock by means of the adaptive mechanism or the like.
Thus, the regenerated user data received from the user terminal is recovered using the transmitting clocking by means of a method closing the time loop at the interface to a synchronized network when the user data is transmitted over an asynchronous packet network to a terminal entity and new data is returned in a synchronous fashion and transmitted back to said interface where timing and data are merged together. Thereby, the method is offering an interface to the synchronized network that is fully in compliance with existing synchronous interfaces, by providing a network interface that is closing the time loop locally and merging it with the user data from the external terminal entity.